U.S. Pat. No. 3,926,602 describes a process for preparing glass-ceramic articles possessing highly-durable, integral surface layers demonstrating distinctly metallic lusters. When those surface layers were developed in situ upon previously polished glass surfaces, mirror-like finishes with good color and spatial reproduction of reflected images could be produced.
The articles were prepared from glasses consisting essentially, expressed in weight percent on the oxide basis, of about 0.5-3.5% FeO, 3-6% Li.sub.2 O, 16-21% Al.sub.2 O.sub.3, 65-75% SiO.sub.2, and 1.5-7% RO.sub.2, wherein RO.sub.2 consisted of 1.5-6% TiO.sub.2 and 0-3% ZrO.sub.2. Two different types of crystallization were obtainable where two different heat treatment schedules were employed.
Thus, in the first schedule, the glass articles were subjected to temperatures between about 850.degree.-1025.degree. C. to cause the growth of beta-quartz solid solution crystals in the interior portion of the article and, at the same time, a very thin integral surface layer containing alpha-hematite crystals was formed. The articles displayed a varying degree of transparency, depending upon the amount of iron included in the base composition.
In the second schedule, the glass article was heated to at least about 1050.degree. C. which caused conversion of the beta-quartz solid solution crystals to beta-spodumene solid solution with the article becoming opaque as a consequence. However, the thin surface layer of alpha-hematite crystals remained intact.
U.S. application Ser. No. 757,582, filed concurrently herewith by the present applicants, discloses the manufacture of glass-ceramic articles wherein beta-quartz solid solution and/or beta-spodumene solid solution crystals constitute the predominant or sole crystal phase in the interior portion of the articles, but wherein a thin integral surface layer containing magnetite crystals is developed in situ. The method of the invention involves two basic steps.
First, a glass article consisting essentially, expressed in weight percent on the oxide basis, of about 1-6% FeO, 3-10% Li.sub.2 O, 15-40% Al.sub.2 O.sub.3, 40-75% SiO.sub.2, and 2-7% RO.sub.2, wherein RO.sub.2 consists of 2-6% TiO.sub.2 and 0-3% ZrO.sub.2, is subjected in an oxidizing environment to a temperature between about 750.degree.-1025.degree. C. to cause the growth in situ of beta-quartz solid solution crystals in the interior portion of the article and, simultaneously, a thin integral surface layer containing alpha-hematite crystals is developed in situ. Where a crystallization temperature in excess of about 1050.degree. C. is employed, the beta-quartz solid solution crystals will be converted to beta-spodumene solid solution crystals. The thin integral surface layer containing alpha-hematite crystals will not be disturbed.
Subsequently, the crystallized article is exposed to a H.sub.2 O-containing reducing atmosphere at a temperature between about 450.degree.-650.degree. C. to cause the reduction of the alpha-hematite crystals in the surface layer to magnetite. In the preferred practice, the crystallized article is initially contacted with a hot mineral acid and then exposed to the reducing environment. Wet forming gas provided the most desirable reducing atmosphere.
U.S. application Ser. No. 757,584, filed concurrently herewith by the present applicants, discloses the manufacture of glass articles having very thin, integral surface layers containing hematite crystals (alpha-Fe.sub.2 O.sub.3), or magnetite crystals (Fe.sub.3 O.sub.4), or metallic iron crystals dispersed within a glassy matrix. The crystal content of the surface layers is very high, i.e., greater than about 50% by volume, such that the crystals provide an essentially continuous film.
The method of that invention for achieving a glass having an integral surface layer containing hematite crystals comprehends two basic steps:
(1) a batch for a glass consisting essentially, in weight percent on the oxide basis, of 1-15% R.sub.2 O, wherein R.sub.2 O consists of 0-10% Li.sub.2 O and 0-15% Na.sub.2 O and/or K.sub.2 O, 0.3-13% FeO, 15-35% Al.sub.2 O.sub.3, 55-80% SiO.sub.2, and O-5% TiO.sub.2 and/or ZrO.sub.2, is melted and formed into a glass article of a desired configuration; and PA1 (2) the glass article is heat treated at a temperature of at least 675.degree. C., but less than 950.degree. C., under oxidizing conditions to cause the crystallization in situ of hematite crystals in the surface of the glass.
When the glass article having the integral surface layer containing hematite crystals is exposed to a H.sub.2 O-containing reducing atmosphere at a temperature between about 450.degree.-650.degree. C., the hematite crystals are reduced to magnetite.
When the glass article having the integral surface layer containing hematite crystals (or magnetite crystals) is exposed to an essentially dry reducing atmosphere at a temperature between about 450.degree.-650.degree. C., the hematite crystals (and magnetite crystals) are reduced to metallic iron.